What happens if a dog ingests visine?

Give lots of water! The active ingredient in Visine eye drops is Tetrahydrozoline HCL 0.05%. Swallowing this substance can result in a number of nasty effects including: lowering of body temperature, making breathing difficult or halting it entirely, blurring vision, causing nausea and vomiting, elevating and then dropping blood pressure, causing seizres or tremors or even sending the ingester into a coma. It causes depression of the central nervous system. You should seek medical attention if the dog ingested more than 2mL of the eye drops.